Certain aerospace components such as pitot probes, pitot-static probes, temperature sensors, and other air data sensors or probes have a critical need to efficiently transfer heat from internally placed heater coils to the exterior of the component during anti-icing or deicing procedures. For that reason, many of these components are typically made from nickel or high nickel content alloys. Although nickel generally demonstrates relatively good corrosion resistance, over time, such nickel components do exhibit some corrosion and subsequent erosion due to constant exposure of moisture, oxidants, halide salts, sulfur-bearing gases, soot, particle impingement, cleaning fluids and many other corrosive solutions.
Because of the importance of air data sensors to aircraft navigation and control, there has been much effort directed to the design and fabrication of air data sensors that are both small and reliable. Various types of air data sensors are disclosed, for example, in U.S. Pat. Nos. 4,730,487; 4,615,213; 4,522,070; and 4,378,696 the disclosures of which are herein incorporated by reference. There remains however, a continuing need for air data sensors or probes that retain thermal conductivity and more importantly exhibit improved corrosion resistance properties.
It is well known that chromium is often used in order to enhance the strength and corrosion resistance of various metal components. Chromium surface treatments, such as chromium coatings, often improve the reliability, maintainability and quality of many products by providing resistance to corrosion, thermal insulation, as well as environmental shielding. Chromium surface treatments on various substrates are also practiced in order to realize a more decorative surface appearance in addition to providing a corrosion or wear resistant surface.
Electroplating of chromium is the most utilized form of surface treatment involving chromium. Chromium plating includes both decorative chromium plating as well as hard chromium plating. Hard chromium plating differs from the decorative plating, mostly in terms of the thickness of the chromium applied. Hard chromium plating may be ten to several hundred .mu.m thick, whereas chromium layers used in the decorative plating process may be as thin as 0.25 .mu.m. Hard chromium plating is noted for its excellent hardness, wear resistance and a low coefficient of friction. Decorative chromium plating on the other hand retains its brilliance because air exposure or other oxidizing exposure immediately forms an invisible protective chromium oxide film.
Chromizing is another principal method of obtaining a chromium rich surface on substrates. In a typical chromizing process, chromium is diffused into the metal substrate from a packed bed in a furnace at about 1100.degree. C. to produce an effective combination or solid solution of the chromium and the substrate metal or alloy being chromized. The thickness of the chromium surface treatment can be controlled by the length of time of the furnace treatment. Chromizing is an economical process that improves corrosion resistance of many components, such as turbine blades, where the appearance is typically not an important consideration. However, chromizing typically does not yield a surface morphology that is aerodynamically smooth enough for external air data sensors.
Other less utilized chromium surface processes include ion implantation, sputtering, chemical vapor deposition, physical vapor deposition and metal spraying. The related art concerning ion implantation processes are described in articles by Sioshansi "Ion Beam Modification of Materials For Industry", Thin Solid Films, Vol 118 pp 61-71 (1984) and Brown et al. "Novel Metal Ion Surface Modification Technique", Appl. Phys. Lett., Vol 58, No. 13 (1991). As discussed in the Sioshansi article, ion implantation of chromium has previously been used for improving the service life and performance of precision steel bearings.
Very generally, ion implantation is a process for electrically injecting atoms of one element into a selected target of another material and more particularly a process for injecting the ions to selected depths and in selected concentrations in order to produce an alloy or other solid mixture having a different composition from the original target material and therefore exhibiting different and sometimes highly desirable chemical and physical properties.
Ion plating, on the other hand, is a process whereby a coating material is evaporated in the plasma region of a gas discharge, ionized, and physically accelerated toward the substrate under influence of electric fields. The ionized particles are deposited on clean surfaces of the substrate with high energy to penetrate the surface and form coatings with uniform thickness, exhibiting excellent adhesion characteristics. The ion plating process is typically carried out in a low pressure chamber which has two properly spaced electrodes. When more energy is available via an applied voltage than the ionization energy of the gas molecules, a gaseous discharge takes place and current can be carried between the electrodes. Electrons ejected from a negative cathode are accelerated toward the positive anode, gaining energy from the electrical field. As the electrons travel toward the anode, some collide with gas molecules, giving up part of their energy to produce positive ions and extra free electrons as well as visible light. The heavy, slow moving positive ions remain in the space between the electrodes longer than the lighter high velocity electrons, giving a net positive space charge which in turn tends to further accelerate electrons from the cathode to produce a self-sustaining glow discharge.
If an ionizable material is evaporated in the plasma region of the discharge, while the above discharge is taking place, many of the evaporant atoms are struck by electrons and become ionized. They can then be accelerated to a second cathode. If the substrate to be coated is made the cathode, the positively charged evaporant ions follow electric field lines to impinge on the substrate surface with energy in electron-volts very nearly equal to the full anode to cathode potential. The evaporant ions are applied to the substrate surface and, at the same time, ionized gas atoms are removing material from the substrate surface by sputtering.
Nickel--chromium alloys will form a solid solution up to about approximately 30% weight chromium. Chromium is often added to nickel to enhance the strength, corrosion resistance, oxidation resistance, hot corrosion resistance and electrical resistance of the various components. In combination, nickel and chromium have been used to form nickel-based superalloys, such as Nichrome type alloys, which are well known and often utilized materials. These nickel-based superalloys, however, do not exhibit the preferred mechanical properties suitable for manufacturing air data sensors or probes. Compared to pure nickel, the nickel--chromium alloys also exhibit greatly reduced thermal transport properties and are less suitable for air data sensors or probes.
Accordingly, there is a need for an improved and reasonably economic method of applying chromium surface treatments to air data sensors and probes. The improved chromium surface treatments should offer improved corrosion resistance during operational use of the air data sensor or probe but allow the air data sensor or probe to retain the preferred mechanical properties for workability offered by conventional nickel-based air data sensors and probes. More importantly, the chromium surface treatment should be an economically feasible process.